Field of the Invention
The present invention relates to a luminance control device, a display device comprising the same and a method of controlling luminance of a display device.
Discussion of the Related Art
The pixels of an organic light-emitting diode (“OLED”) display comprise organic light-emitting diodes (“OLEDs”), which are self-emissive. As shown in FIG. 1, an OLED is composed of a stack of organic compound layers including a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL, with all the layers situated between an anode and a cathode. The OLED display reproduces input images based on a phenomenon in which electrons and holes recombine in the organic layer in the OLED of each pixel to emit light as current flows through a fluorescent or phosphorescent organic thin film.
The OLED display can be classified into many types, depending on types of emissive material, emission methods, emission structures and driving methods. The OLED display can be classified as a fluorescent emission device or a phosphorescent emission device depending on emission methods, or classified as a top emission device or a bottom emission device depending on emission structures. Also, the OLED display can be classified as a PMOLED (Passive Matrix OLED) display or an AMOLED (Active Matrix OLED) display depending on driving methods.
To efficiently reduce power consumption of a display device, it is beneficial to lower screen brightness. Simply turning down brightness can reduce power consumption, however, it may degrade picture quality. If the user turns down the brightness of display images, especially in a high-temperature range, not in a room-temperature range, the brightness of images with a high average picture level (hereinafter, “APL”) may become too low. APL is defined as the average brightness of the brightest color in 1-frame image data.
At room temperature, the higher the APL, the more the bright pixel data relative to the total amount of pixel data. By contrast, the lower the APL, the more the bright pixel data relative to the total amount of pixel data. For 8 bits of pixel data, the peak white gray level is “255.” On the other hand, in a high-temperature range, where the brightness is much lower than the brightness at room temperature, the peak white gray level is lower than “128.”
As depicted in FIG. 2, when about 10% of the pixels across the entire screen displays the peak white gray level and the other pixels display the black gray level 0, the APL is 10%. By contrast, when the pixels across the entire screen display the peak white gray level 255, the APL is 100%. In what follows, the peak luminance is defined as the luminance at 10% APL, and the full white luminance is defined as the luminance at 100% APL.
In peak luminance control (hereinafter, PLC), based on a variation with APL of luminance shown in FIG. 3 and PLC curve shown in FIG. 4, if the APL is low, the peak luminance can be turned up by increasing the PLC gain, and if the APL is high, the peak luminance can be turned down by decreasing the PLC gain.
Based on the PLC curve, when the APL is 10%, the peak luminance is turned up by increasing the PLC gain. The peak luminance gain is 100%. When the APL is 10%, the peak luminance is kept at 600 nits. Based on the PLC curve, when the APL is 100%, the peak luminance is turned down by decreasing the PLC gain. The peak luminance gain is 66.6%. At 100% APL, the peak luminance is adjusted from 600 nits to 400 nits.
As stated above, the brightness of display images is kept within a certain range as the peak luminance varies with increase or decrease in APL at room temperature.
Meanwhile, as shown in FIG. 5, temperature brightness control (hereinafter, “TBC”) for controlling brightness by temperature is put into operation, along with PLC control, at a particular temperature higher than room temperature, to control maximum brightness.
Since both PLC control and TBC control are put into operation at a high temperature of 85° C.—a specific temperature higher than room temperature, maximum luminance is adjusted to a lower level than it was originally intended. That is, maximum luminance may be brought down to a level lower than 50% of what could be achieved at the high temperature of 85° C.